1. Field of the Invention
The present invention disclosed in this specification relates to a method for manufacturing a semiconductor layer.
2. Description of the Related Art
In recent years, a technique for forming a semiconductor element such as a thin film transistor (TFT) having a semiconductor film (e.g., the thickness of about several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface such as glass has attracted attention.
Thin film transistors are widely used in electronic devices such as ICs and electro-optical devices, and their development especially as switching elements for image display devices has been accelerated.
Instead of silicon wafers that are manufactured by thinly cutting an ingot of a single crystal semiconductor, semiconductor substrates called silicon-on-insulator (SOI) substrates have been developed, which have a thin single crystal semiconductor layer over an insulating layer and are becoming widely used as substrates for manufacturing microprocessors or the like.
This is because an integrated circuit using an SOI substrate has attracted attention as a circuit which reduces parasitic capacitance between a drain of a transistor and the substrate, improves performance of a semiconductor integrated circuit, and reduces power consumption.
As a method for manufacturing an SOI substrate, a hydrogen ion implantation separation method is known. For example, ions of hydrogen or the like are implanted to a silicon wafer provided with a silicon oxide film on its surface. Accordingly, a microbubble layer can be formed at a predetermined depth from the surface of the silicon wafer. This microbubble layer serves as a cleavage plane, and a thin single crystal silicon layer is bonded to another silicon wafer. Then, the single crystal silicon layer is separated from the cleavage plane by heat treatment to form an SOI substrate.
In addition, there is an attempt to form an SOI layer over an insulating substrate such as a glass substrate. As an example, a method in which a thin single crystal silicon layer is formed over a glass substrate by an ion implantation separation method is known (see Reference 1: Japanese Published Patent Application No. 2004-87606).